With the increasing demand for precision-machined parts, a variety of positioning devices have recently been developed and put to use to meet the demand. A conventional positioning device will first be explained with reference to FIGS. 1 and 2 which illustrate a cam-type lathe incorporating the device. A headstock 1 includes a main spindle 2 fitted with a chuck 3 adapted to rotate a workpiece 4. A tool 5 is secured by a tool holder 6 and a tool mount 7 to a table 9 adapted to move in opposite X-directions. Table 9 and a table 10, adapted to move in opposite Y-directions, are driven respectively by cams 13 and 14 through links 11 and 12. A stop 15 is provided to limit the position of the table 10.
As illustrated in FIG. 2 table 9 and table 10 are each so designed that a movable section 16 thereof shifts along a guide 17. Because the sliding movement of the movable section 16 of the table relative to the guide 17 must be suitably accurate, it is common practice to interpose contrivances such as a sliding member 18 and a screw 19 associated therewith to apply a preliminary pressure as shown in FIG. 2. However, if a preliminary pressure is applied to the sliding member 18, a large frictional force is generated between the guide 17 and the movable section 16 of the table to cause a "stick slip" which leads to decreased accuracy.
Moreover, when the stop 15 is utilized to limit the position of table 10, the stop is caused to undego elastic deformation by the force acting thereon from the table 10, and this means that the force with which table 10 abuts the stop 15 must be constant in order to maintain constant the amount of elastic deformation. Thus, the friction force generated between the movable section 16 and the guide 17 influences the magnitude of forces with which the table 10 contacts the stop 15, with the result that positioning accuracy is adversely affected.
It may be contemplated to employ balls or rollers in lieu of sliding member 18 to reduce the friction force by way of a rolling mechanism, but such an arrangement has been found to be of little practical value for achieving high-precision positioning because of dimensional variation of the balls or rollers.